1. Field of the Invention
The present invention relates to a system of transmitting an information signal such as an underground information signal suitable for use in drilling a crude petroleum well or a gas well for example, wherein the information signal is transmitted to the ground in real time.
2. Description of the Related Art
In recent years, an MWD (Measurement While Drilling) system for transmitting stratum information and drilling information to the ground in real time while drilling has been used to effect a reduction in drilling cost and an improvement in safety while and immediately obtaining the drilling information so that drilling control is effected. The details of the MWD technology have been described in, e.g., a Journal, Vol. 112, No. 11, pp. 877-884 (issued in 1992) published by IEE of Japan.
FIGS. 1, 2 and 3 are respectively general views for describing a conventional system for transmitting a signal. In the drawings, reference numeral 101 indicates a drilling rig provided on the ground. Reference numeral 102 indicates a drill pipe supported by the drilling rig 101 and rotated in a pit well 100. The drill pipe 102 has a predetermined length (of about 9 meters, for example) and screws attached to both ends thereof. As the pit well 100 becomes deeper upon drilling, subsequent drill pipes 102 are successively added to the initial drill pipe 102 so that the overall length can be increased.
A drill collar 102a to be connected with the drill pipes 102 is connected to a leading end portion of a train or sequence (called a "drill string") of the so-added drill pipes 102 as shown in FIG. 2. Further, a drill bit 102b, which serves as a cutting blade, is mounted to a leading end portion of the drill collar 102a and drills the pit well 100 under the rotation of the connected drill pipe 102.
Reference numeral 103 indicates a mud pump for feeding water (called "mud") mixed with mud into the entire drill pipe 102 from the ground. Reference numeral 103a indicates a mud tank for storing muddy water therein. The muddy water in the mud tank 103a is fed downward under pressure into the drill pipe 102 by the mud pump 103. After the muddy water has reached to the drill bit 102b, it travels or proceeds to the ground through an outer space between the pit well 100 and the drill pipe 102 and is returned to the mud tank 103a again.
Reference numerals 105a through 105n respectively indicate various detectors for detecting information required to drill a bottom of pit. These detectors are mounted to the leading end portion of the drill collar 102a and detect the following physical quantities:
(1) load of drill bit, torque of drill bit, bending moment, vibrational values, etc. as drilling information: PA1 (2) bearing (for evaluating whether or not a pit well is being drilled in a predetermined direction), inclination, pressure, temperature, etc. as pit-well information: PA1 (3) stratum gamma-ray density or level, stratum resistivity (electrical resistance), etc. as stratum evaluation information. PA1 (1) The piezoelectric ceramic is weak in strength as compared with a metallic material. PA1 (2) The piezoelectric ceramic has a Curie temperature of about 120.degree. C. Since piezoelectric distortion is not produced when the Curie temperature of the piezoelectric ceramic exceeds 120.degree. C., the piezoelectric ceramic cannot be used under a high-temperature environment. PA1 (3) When a magnetic-field applying direction coincides with a crystal distortion axis and a pre-load for applying pressure in advance is put on the piezoelectric ceramic, a stress is imposed on electrodes so that the electrodes are apt to suffer damage. PA1 (4) An oscillating frequency of the piezoelectric ceramic is fixed depending on dimensions such as the thickness of the piezoelectric ceramic, etc. That is, it is necessary to carry out excitation under the oscillating frequency corresponding to the dimensions. It is indispensable to efficiently inject impactive energy enough to generate an acoustic wave into the pipe. PA1 (5) Even when the exciting voltage is modulated at the repeat rate, the carrier wave is required. Since the transfer of energy is carried out under the vibration coupling made through the coupling unit, the excitation based on the carrier wave places a limitation on the transfer of the energy to the pipe. PA1 (6) In a signal transmission system based on the repeat rate, it is necessary to repeatedly generate a burst signal in order to make a decision as to the repeat rate. The minimum time required to generate and transfer information per bit is restricted, so that an increase in bit rate becomes hard.
Reference numeral 104b indicates a modulator attached to the leading end portion of the drill collar 102a, for modulating various signals detected by the detectors 105a through 105n, which are supplied from the bottom hole during drilling and transmitting the modulated signals to a transmitter 104. Reference numeral 104 indicates the transmitter for amplifying the signals supplied from the modulator 104b and outputting the amplified signals to a transmitting antenna 104c.
Reference numeral 104a indicates a generator for supplying operating power to the modulator 104b and the detectors 105a through 105n or the like. Reference numeral 104g indicates a moving or rotor blade of a turbine, which rotates according to the flow of muddy water. The generator 104a is rotatably driven under the rotation of the moving blade 104g so as to generate power. Reference numeral 104h indicates a stationary or stator blade of the turbine, for changing the direction of the flow of the muddy water.
A transmission unit (including the transmitter 104, the modulator 104b, etc.) and the detectors 105a through 105n are accommodated within a storage container 104f provided within the drill collar 102a and are sealed so as to avoid the influence of the muddy water. Further, the generator 104a and the turbine are also coupled to an upper portion of the storage container 104f.
Reference numeral 104c indicates the transmitting antenna attached to a part of an outer peripheral portion of the drill collar 102a with both separated by an insulating material 104e. The transmitting antenna 104c is electrically coupled to the transmitter 104 with a bolt 104d insulated by and attached to the drill collar 102a so as to extend through the drill collar 102a.
Reference numeral 106 indicates a receiving antenna mounted on the ground. Reference numeral 107 indicates a receive amplifier for amplifying a signal received by the receiving antenna 106. Reference numeral 108 indicates a demodulator for demodulating a signal sent from the receive amplifier 107. Reference numeral 109 indicates a data processor for performing signal processing such as A/D conversion on a signal supplied from the demodulator 108 and thereafter performing data processing such as a storage process, a computational process on the value of the so-processed signal. Designated at numeral 110 is a display for displaying data sent from the data processor 109 thereon or indicating a warning thereon.
The operation of the conventional signal transmission system will now be described below. The drilling of petroleum or gas well is carried out by successively connecting the drilling rig 101, the drill pipe 102 and the drill collar 102a to one another and rotating the drill bit 102b attached to the leading end portion of the drill collar 102a. During drilling, on-drilling muddy water stored in the mud tank 103a is sent out from the mud pump 103. Further, the muddy water circulates in the mud tank 103a through the drill pipe 102, the drill bit 102b and a pit wall to thereby deliver drilled debris to the ground.
The transmitter 104 is provided in the bottom hole. When the on-drilling muddy water is fed from the mud pump 103 and reaches a predetermined flow rate, the generator 104a mounted to the transmitter 104 is driven and started up by the on-drilling muddy water so as to supply power to the transmitter 104. When the power is supplied to the transmitter 104, data detected by the detectors 105a through 105n and modulated by the modulator 104b are outputted to the transmitting antenna 104c from the transmitter 104. Thereafter, the data are transmitted to the inside of the stratum through the transmitting antenna 104c in the form of a radio wave. The radio wave is received by the receiving antenna 106 provided on the ground and subjected to filtering and amplification by the receive amplifier 107, after which the so-processed radio wave is input to the demodulator 108. The demodulator 108 effects a demodulating process on the input signal and demodulates the so-processed signal in the form of the data detected by the detectors 105a through 105n. The demodulated data are input to the data processor 109 where the input data are checked against ground information such as time, depth, etc. and effects data processing according to display purposes. Thereafter, the processed data are displayed on the display 110.
As has been described in, e.g., "TELEMETRY USING THE PROPAGATION OF AN ELECTROMAGNETIC WAVE ALONG A DRILL PIPE STRING" (MWD SYMPOSIUM PROCEEDINGS pp 49-51, FEB. 26-27 1990, LOUISIANA STATE UNIVERSITY, LOUISIANA) by R. GRUDZINSKI et al., the conventional system for and method of transmitting the radio signal are accompanied by drawbacks that the radio wave is relatively so attenuated in the ground, the achievable distance of the radio wave is short and the radio wave is rendered unfit for the use of a deep well having a distance of 600 meters or so.
In order to solve such drawbacks which arise when the radio wave is used as a signal transmission medium, there has been proposed an acoustic wave type pipe transmission system using a piece of piezoelectric ceramic as a signal transmission source as has been described in EP-A1-0552833.
FIG. 4 is a side view showing the structure of the pipe transmission system disposed in the bottom hole. In the drawing, reference numeral 332 indicates an oscillator or a signal generator using a piezoelectric ceramic element. Reference numeral 334 indicates a receiver sub on the receiving side. Reference numeral 335 indicates a receiving transducer for converting a received acoustic wave into an electric signal. Reference numeral 319 indicates an MWD tool and reference numeral 311 indicates a drill pipe. Designated at numeral 312 is a drill collar.
The operation of the pipe transmission system will now be described. An ultrasonic wave generated from the signal generator 332 is sent to a tubular body or a pipe made up of the drill collar 312 and the drill pipe 311 and is propagated upward. In the conventional example, the ultrasonic wave is received by the receiving transducer 335 on the receiver sub 334 disposed in the course of the pipe. Further, information is transmitted to the ground through the MWD tool 319 in accordance with a method using a mud pulse, for example.
FIG. 5 is an exploded perspective view showing the structure of the signal generator 332. In the drawing, reference numeral 372 indicates a crystal of stacked ceramics. FIG. 6 is a cross-sectional view of the signal generator 332. Reference numeral 353 indicates an elastic body such as a spring or the like. Reference numeral 337 indicates a coupling unit for coupling the signal generator 332 to the pipe. The signal generator 332 is provided in a concave portion defined in the pipe. The signal generator 332 has such as a structure that one end of the coupling unit 337 is pressed against a transverse surface of a drill string and the elastic body 353 applies a biasing force to the crystal 372 so as to cause vibration coupling between vibrations of the signal generator 332 and the pipe.
FIGS. 7(1) and 7(2) are respectively waveform charts for describing the waveform of a drive voltage generated by the signal generator 332 employed in the conventional example shown in FIGS. 4 through 6 and the waveform of a signal propagated through the pipe. Reference numeral 366 indicates a waveform of the drive voltage to the signal generator 332. Reference numerals 367 and 368 respectively indicate the propagated waveforms generated in the pipe.
The signal propagated through the pipe is generated as follows. That is, a carrier wave of a frequency of about 20 kHz corresponding to the resonance frequency of the signal generator 332 is first applied to the ceramic crystal 372 as a four-wave burst voltage so as to excite the ceramic crystal 372. Vibrations of the excited ceramic crystal 372 are then transferred to the pipe via the coupling unit 337, so that an ultrasonic vibration comprised of a longitudinal wave 367 and a transverse wave 368 is created in the pipe.
FIGS. 8(1) and 8(2) are respectively waveform charts for describing modulation of exciting voltage at the signal generator 332. FIG. 8(1) shows the waveform of an exciting voltage signal at the time that a bit is "1". FIG. 8(2) illustrates the waveform of an exciting voltage signal at the time that the bit is "0". Repeat rate of the signals indicates binary code. A first rate corresponds to the bit of "1" and is 6.2 msec in the conventional example in FIG. 8(1), whereas a second rate corresponds to the bit of "0" and is 12.4 msec in the conventional example in FIG. 8(2).
The propagated ultrasonic vibration generated from the signal generator 332 is propagated upward through the drill string and detected by the receiving transducer 335 having the same structure as that of the signal generator 332, so that an output voltage is produced according to vibrations of a piezoelectric crystal of the receiving transducer 335. As an alternative to the receiving transducer 335, a piezoelectric accelerometer is used for the detection of the ultrasonic vibration.
The sound signal detected by the receiving transducer 335 is converted into an electric signal. Thereafter, noise components are removed from the converted electric signal by an unillustrated filter. Further, the noise-removed signal is converted into a digital signal by an unillustrated A/D converter so as to be input to the MWD tool 319. Thereafter, this signal is transmitted to a further upper portion base on a mud pulse, for example.
Since the conventional system for transmitting a signal is constructed using the piezoelectric ceramic in such a manner that the piezoelectric effect of the piezoelectric ceramic is used to generate the acoustic signal, the following problems arise: